The invention relates to a non-destructive method of examining and testing an electric device such as an integrated or printed circuit, or a transducer using the same technologies. Using this method, radiation emitted by the device itself after stimulation is detected by scanning, and electric signals characteristic of the detected radiation are formed. These signals are then compared with reference signals.
A large number of existing micro-electronic devices are made by the integrated circuit technique, basically by lithography, chemical etching (photogravure) and metal-coating of slices or chips of silicon or of compounds III-V and II-VI in the periodic table of the elements. Arrays of photo-detectors are examples of such devices. These examinations can also be applied to single or multiple-layer printed circuits, chips cut from drawn bars of silica or compounds III-V and II-VI in the periodic table of the elements, or to microwave circuits obtained by methods similar to those for integrated-circuit chips.
Numerous efforts have already been devoted to automatic examination for faults in these kinds of electronic devices and transducers. Usually the examination is made by optical detection of radiation in the visible or infrared range, generated or reflected by the device. In U.S. Pat. No. 3,803,413, for example, an excitation signal (either a direct voltage or a modulated or pulse voltage) is applied to a printed circuit, and the infrared radiation emitted by the various circuit components is detected by a scanning mechanism. The radiation density at each point of the circuit is then compared with a reference so as to detect any faults associated with Joule-effect heating of the various metal coatings making up the circuit.
There are also methods using the technique of optical correlation of images by analysing the optical radiation reflected by the surface of the circuit. This applies particularly to European patent specifications Nos. 66321 and 66694.
The first of the aforementioned processes detects faults mainly relating to the dimensions of the components. Processes using reflection of optical radiation can detect faults in the geometry of the circuit or stresses occurring at the surface, e.g. as a result of distortion of the substrate or chip.
On the other hand these inspection processes cannot detect faults in the structure, inter alia at the interfaces between the various layers or faults in the substrate. Such faults are not inevitably shown by abnormal infrared radiation during transit of a given electric signal or by simple reflection or scattering of incident radiation.
It has already been proposed to use a scanning electron microscope (SEM) for forming an image by picking up electrons emitted as a result of electron scanning. This method has a number of disadvantages, particularly for production tests, since the electric devices under test have to be placed in a vacuum chamber and covered with a deposited layer of metal, which means that the process cannot be applied rapidly and systematically to a large production series. This method of irradiation cannot be used for simultaneous electric testing at speeds compatible with manufacturing rates. This kind of examination is described inter alia in U.S. Pat. No. 4,358,732 and French patent specification No. 2 058 756. In the latter document, electric pulse tests are made simultaneously and in synchronism with electron bombardment. This type of test cannot locate structural faults but only gives information about the performance of the tested circuit; it is also slow.
GB patent specification No. 2 069 152 also relates to a method of testing integrated circuits, in which the circuit is supplied with a voltage set to a value slightly higher than the marginal voltage at which faults in the circuit cause voltage deviations at the output when a logic test is applied to it. When a beam of radiation scans the surface, faults are revealed in the characteristic of the resulting photoelectric current and can thus be detected, but not located. This process does not comprise displaying faults by using the radiation re-emitted or transmitted by the circuit, and consequently no structural faults can be located.
It is known that if discontinuities are present in material excited by incident radiation, there will be physical interaction between the radiation and the discontinuities, resulting in a change in the mode of excitation at the interfaces.
When an incident beam strikes a semiconductor or an interface, it produces electron-hole pairs. If these mobile charge carriers reach or come from the p-n junction depletion region, they are swept by the junction potential, producing an external inverse current which is superposed on the current induced by the external electric test voltage. This current can be collected and amplified to obtain information about the electric state of the junctions through which it passes, or variations in the junction characteristics.
In addition, when a structure comprising a number of layers having different electric conductivity, in which electron transport occurs in at least one layer, is excited by incident radiation, the electron transport stimulates the change in the mode of excitation which occurs at the structure layer interfaces and is revealed by induced radiation (called secondary radiation). The signal response is also modified by the interaction between the radiation and the electric signal producing the electron transport.
The invention aims to take advantage of these physical phenomena in detecting faults in the structure of electronic devices at the interfaces (including interfaces between metal coatings and substrate) and some faults inside the substrates themselves.